X-ray methods have been used to investigate the solid-state structures of wholly aromatic thermotropic copolyesters prepared from equimolar amounts of p-hydroxybenzoic acid, biphenol and thiophenyl- or oxyphenyl terephthalic acid. We have also compared these structures with that of the unsubstituted copolymer. The scattering data for melt-spun fibres show that the presence of the side chains results in considerably less lateral order. Non-periodic layer lines are observed, and their positions are reproduced in simulated scattering patterns for extended chains of completely random sequence. For the unsubstituted copolymer, the chains are not fully extended but are contracted by approximately 1%, probably due to the requirements to pack random sequences with three-dimensional order. A further contraction of 1.5% occurs for the backbones of the substituted copolymers, probably in an effort to minimize the free volume. Nevertheless, the presence of the side chains results in a reduction of the density by similar to 20%. The substituted copolymers adopt a distorted structure similar to that seen for poly(phenyl-p-phenylene teraphthalate), in which the side groups on adjacent chains are interdigitated. This is possible even for random sequences because there is only one side chain for every four aromatic units along the backbone. Nevertheless, this interdigitation appears to preclude any registration of adjacent random sequences, and thus there is no evidence for the three-dimensional order observed for the unsubstituted copolymer.